High-resolution seismic surveys carried out on the shelf off Trabzon (southeastern Black Sea) show that the sub-bottom stratigraphy consists of two main depositional sequences (A and B) one of which contains zones of acoustic turbidity. Of these, the upper depositional sequence (A) is thought to comprise Holocene sediments, while the lower depositional sequence (B) is interpreted as approximating to the Pleistocene. The boundary between these sequences is defined by a reflector (R), which is interpreted as the pre-Holocene erosional surface. The acoustic turbidity observed on seismic profiles is interpreted as representing gas accumulations in the sediments of the upper depositional sequence. Previous geochemical investigations in the area indicate that acoustic turbidity implies biogenic methane gases in bubble form. On the basis of published hypotheses on the generation of the methane gases, it is suggested that the upper boundary of the zone of acoustic turbidity on seismic profiles in this study corresponds to the boundary between a biogenic sulfate-reducing zone and the underlying carbonate-reducing zone. Apart from acoustic turbidity, some bright spot anomalies which are also interpreted to arise from the presence of methane gases within the upper depositional sequence are observed on the seismic records collected from the shelf off Trabzon. (C) 1999 Elsevier Science Ltd. All rights reserved.